1. Technical Field of the Invention
The present invention relates generally to a fuel injection system for an internal combustion engine. More particularly, the invention is directed to a fuel injection system which is operable to regulate the amount of intake air introduced from a portion of an intake passage upstream of a throttle valve to a fuel injection valve provided for each cylinder and the timing where the intake air introduced is ejected to the fuel injection valve for optimizing atomization of the fuel injected from the fuel injection valve over a wide range of engine operation.
2. Background Art
Japanese Patent Second Publication No. 57-54624 teaches an improved fuel injection system which includes an air control valve arranged to introduce a portion of intake air, substantially under the atmospheric pressure, upstream of a throttle valve installed in an intake air passage to an injection nozzle of a fuel injection valve through an air passage bypassing the intake air passage. The air control valve is opened in synchronization with an operation of the fuel injection valve when the throttle valve is somewhat closed, so that the portion of the intake air is ejected around the injection nozzle to mix with the fuel injected by the fuel injection valve for promoting atomization of the injected fuel.
With the above arrangement, even when a flow velocity of the intake air in an intake port of the engine is relatively low such as during idle modes of engine operation, the fuel injected from the fuel injection valve becomes atomized sufficiently, thereby improving combustion conditions of air/fuel mixtures in a combustion chamber of the engine to reduce the amount of harmful emission products such as hydrocarbons (HC) or carbon monoxide (CO) remaining in exhaust gases. In the following discussion, the air mixing means, as mentioned above, will be referred to as timing-controlled air mixture system.
In order to meet emission regulations of automotive internal combustion engines which have become more severe in recent years, an individual injection type of fuel injection system including a fuel injection valve for each cylinder of a multi-cylinder engine, has been put to practical use. In a combination of the timing-controlled air mixture system and the individual injection type of fuel injection system, there has been proposed a single air control valve which is arranged to distribute a portion of the intake air to the fuel injection valves mounted in intake ports of cylinders of the engine through intake air passages, respectively bypassing an intake air passage leading to the intake ports, according to ignition timing of each cylinder, e.g., at each cycle of 180.degree. crank angle in case of a four-cylinder engine.
Such a conventional air control valve of the timing-controlled air mixture system, however, encounters a drawback for the following reasons. When starting the engine at a relatively low ambient temperature, for example, below 0.degree. C. it is necessary to increase an injection period of time a fuel injection valve is open in excess of a crank angle of 180.degree. for assuring stable ignition since the volatility of fuel will be degraded according to a decrease in temperature in the engine. Therefore, when the engine starts at a lower temperature, in a portion of the injection period of time beyond a crank angle of 180.degree. , the intake air is not supplied to the fuel injection valves through the air control valve, thereby causing the degree of atomization of the fuel injected from the fuel injection valves to be reduced greatly. This results in harmful emission products remaining in exhaust gases being increased undesirably.